In recent years, car bodies have been made lighter in weight as means for improving the fuel efficiency due to the environmental problems raised by automobiles and a strong need has arisen for reducing costs by forming parts integrally and streamlining the working processes. High strength hot rolled steel sheet excellent in press workability has therefore been developed. In the past, as such high strength hot rolled steel sheet having a high workability, steel with a mixed structure of a ferrite and martensite structure or ferrite and bainite structure or steel with a substantially single phase structure of mainly bainite or ferrite have been widely known.
In particular, steel of a ferrite and martensite structure has the characteristics of a high ductility and excellent fatigue characteristics, so is being used for automobile wheels etc. For example, Japanese Unexamined Patent Publication (Kokai) No. 6-33140 discloses steel of a ferrite and martensite structure where the amounts of addition of Al and N in the ferrite and martensite structure are adjusted so as to leave solid solution N and obtain a high ageing hardening and thereby obtain a high fatigue strength, but in a ferrite and martensite structure, microvoids form around the martensite from the beginning of deformation and lead to cracking, so there is the problem of poor burring. This made the steel unsuitable for applications such as chassis parts demanding a high burring.
Further, Japanese Unexamined Patent Publication (Kokai) No. 4-88125 and Japanese Unexamined Patent Publication (Kokai) No. 3-180426 disclose steel sheet having a structure mainly comprised of bainite, but since the structure is mainly comprised of bainite, while the burring is excellent, there is little of the soft ferrite phase, so the ductility is poor. Further, Japanese Unexamined Patent Publication (Kokai) No. 6-172924 and Japanese Unexamined Patent Publication (Kokai) No. 7-11382 disclose steel sheet having a structure mainly comprised of ferrite, but similarly while the burring is excellent, hard carbides are made to precipitate in order to secure strength, so the ductility is poor.
Further, Japanese Unexamined Patent Publication (Kokai) No. 6-200351 discloses steel sheet excellent in burring and ductility having a ferrite and bainite structure, while Japanese Unexamined Patent Publication (Kokai) No. 6-293910 discloses a method of production of steel sheet achieving both burring and ductility by use of two-stage cooling to control the ratio of ferrite. However, due to the further reduction in weight, complexity of parts, etc. of automobiles, further higher burring and ductility are sought. Recent high strength, hot rolled steel sheets are being pressed to provide an advance level of workability not able to be handled by the above technology.
Further, Japanese Unexamined Patent Publication (Kokai) No. 2002-180190 discloses an invention relating to high strength hot rolled steel sheet excellent in burring and ductility. While high strength hot rolled steel sheet excellent in the contradictory characteristics of burring and ductility has been obtained, in the hot rolling process, surface defects known as Si scale sometimes occurred resulting in damage to the appearance of the product. Further, high strength hot rolled steel sheet for chassis parts etc. usually is chemically converted and painted after press working. However, problems sometimes arose such as cases of poor formation of the chemical conversion coating (poor chemical conversion) or cases of poor adhesion of the paint after application. These problems are believed to be due to the large amount of Si contained in the steel. In this way, Si is often used for high strength hot rolled steel sheet, but various types of trouble arise.
Further, Japanese Unexamined Patent Publication (Kokai) No. 6-128688 discloses technology for adjusting the hardness of the ferrite phase in a ferrite and martensite structure so as to improve the durability and achieve both ductility and fatigue strength. Further, Japanese Unexamined Patent Publication (Kokai) No. 2000-319756 discloses technology for adding Cu to a ferrite and martensite structure so as to strikingly improve the fatigue characteristics while maintaining the ductility. In both cases, however, to secure sufficient ferrite in the hot rolling process, the amount of Si added becomes high, so in the hot rolling process, surface defects known as Si scale are formed in some cases and the appearance of the product is damaged in some cases. Further, high strength hot rolled steel sheet for chassis parts etc. normally is chemically converted and painted after press working. However, problems sometimes arose such as cases of poor formation of the chemical conversion coating (poor chemical conversion) or cases of poor adhesion of the paint after application.